my list would be heavily k-line. k-line was perfect for me because the price range was right for a teenager (I'm 25 now), and they usually made items like passenger cars in multiple sizes so I could get the smaller sizes for my 4x8 layout while still having the quality they offer in larger sizes. also for engines they usually offered both a conventional and command version which was great because, despite being an aerospace engineer, I like the ease of working with conventional.
I have the "15 broadway limited Madison set, so wish I had picked up the 2 car add-on set which included the baggage car and pershing square Pullman. hard to find now, and when they do show up, the asking price is about what I paid for the original set of 4.
would have loved the empire state express they put out. think it was in one of the last catalogs before they became k-line by lionel, so now those that do show up go for big bucks and the other companies that make them are usually full scale and command, so they wont work for my layout.
the other main item would be something I posted about before. my father and I sold a lionel postwar collection when I was maybe 10 for a friend of a family member. it was offered to us for $2000, but we couldn't really swing it at the time and we just didn't know. the first engine we sold had a super rare master carton so just that first auction ended up going for over $4000. would have liked to have kept some of those items for myself as well